1. Field of the Invention
The present invention relates to a process for producing a copolyester from a lower dialkyl ester of a terephthalic acid (LDE), a glycol (GLY) and a dicarboxylic acid (DA). Using a specific catalyst system of the present invention allows to conveniently produce the copolyester by sequencing an ester interchange and direct esterification. In particular, the catalyst system comprises manganese (Mn), lithium (Li), antimony (Sb) and optionally cobalt (Co). More specifically, manganese and lithium are used as catalysts for the ester interchange of the LDE and GLY, lithium for the direct esterifcation of DA and GLY, while lithium cobalt and antimony are used as catalysts for the polycondensation stage. The manganese is sequestered after the ester interchange. It has been found a copolyester can be produced by adding a slurry of DA/GLY to the reaction product of LDE and glycol of the ester interchange, reacting the DA/GLY slurry by direct esterification therein and copolymerizing the two reaction products.
Furthermore, copolyesters of the present invention are demonstrated to have improved melt flow characteristics and in solid form, improved cutting characteristics during processing and increased tensile strength when bonded in a blend with other polyester fibers.
Copolyesters of the present invention can be used as binder fibers in nonwoven applications or can be used in conjunction with polyester fiber in bicomponent fibers.
2. Prior Art
In prior art processes, copolyesters can be produced by two different routes: ester interchange plus polycondensation, referenced herein as the ester interchange route; or direct esterification plus polycondensation, referenced herein as the direct esterification route. Generally, the ester interchange route can be used with either a batch type process or a continuous process, while the direct esterification route uses a continuous type process.
In the ester interchange route, dimethyl terephthalate (DMT), dimethyl isophthalate (DMI) and ethylene glycol(EG) are typically reacted in the presence of a catalyst (manganese) at atmospheric pressure and at a temperature of from about 180.degree. C. to 230.degree. C. In the presence of the catalyst, these components undergo ester interchange to yield two intermediate monomers and methanol. The reaction which is conveniently done with about 1 mole of DMT, about 0.4 mole of DMI and 2.8 to 3.2 moles of EG, is reversible and is carried to completion by removing methanol formed. During the ester interchange, the two intermediate monomers are the substantial majority product (not considering the methanol), along with small amounts of low molecular weight oligomers.
The monomers are then polymerized by a polycondensation reaction, where the temperature is raised to about 270.degree. C. to about 300.degree. C. and the pressure is reduced to below 1 mm of mercury vacuum and in the presence of a suitable polymerization catalyst (antimony). From this reaction, polyethylene terephthalate/isophthalate) and ethylene glycol are formed. Because the reaction is reversible, the glycol is removed as it is evolved, thus forcing the reaction toward the formation of the polyester.
Manganese is the preferred catalyst for ester interchange reactions, but the amount of manganese employed must be strictly controlled. The presence of too little manganese during the ester interchange reaction results in very long reaction times, while the presence of too much manganese results in unwanted side products during the polycondensation reaction, and unacceptable degradation of the copolyester resulting in poor color (thus lowering the quality of the copolyester). The exact range of manganese which proves to be the most desirable must generally be determined through trial and error because many factors affect the reactivity of the manganese. For example, reaction temperature, reaction pressure, the degree of mixing during reaction, the purity of the raw materials, the presence of other additives, etc., all affect the effectiveness of manganese.
In prior art process, manganese was employed to obtain suitable ester interchange reaction times, but the manganese must be sequestered after ester interchange or during polycondensation by a polyvalent phosphorous compound to aid in reducing the discoloration and unwanted side products. Generally, prior art processes employed about 50 ppm to 150 ppm manganese based on he expected yield of the polymer, as the ester interchange catalyst. Using more than about 150 ppm manganese resulted in polymer degradation even if phosphorous was employed in excess to sequester the manganese. It is believed that this occurred because the phosphorous was incapable of complexing with the manganese to the degree necessary to prevent discoloration.
Known disadvantages for making copolyesters using the ester interchange route starting with a mixture of DMT and DMI are the high price of the DMI and low quality of DMI. Another method to make copolyesters is by direct esterification. By this method, terephthalic acid (TA) isophthalic acid (IPA) and EG are typically reacted without any catalyst in a continuous process. Normally, the TA and IPA are reacted at a pressure of from about 5 psia to 85 psia and at a temperature from about 185.degree. C. to 290.degree. C. These components undergo direct esterification to yield two intermediate monomers and water. The reaction is conventionally done with amounts of TA and IPA corresponding to the desired mixture in the copolyester For example, if a 60/40 (terephthalate/isophthalate) is desired, then 1.2 mole of TA, .8 mole of IPA and from 2.4 to 3.2 moles of EG are reacted.
After the completion of the direct esterification, the monomers are then polymerized by the polycondensation reaction as described in the ester interchange process.
As previously noted, direct esterification is generally conducted in a continuous process. Although this process overcomes the problems resulting from ester interchange due to the use of DMI, other problems occur relating to the use of the continuous process to make copolyester, in particular when relatively small amounts of copolyester are required. Continuous processes are cost effective to operate when relatively large amounts of polyester or copolymer are required. Smaller lot sizes of either polyester or copolyester are produced more cost effectively by a batch process. To this end, the present invention provides a process that can be made using the batch process without the use of dimethyl isophthalate. Furthermore, the present invention may also be used in a continuous process.
One use of copolyesters is in thermally bonded fibrous nonwoven applications such as medical face masks wherein polyester fibers are thermally bonded to copolyester binder fibers that have been blended with the polyester fibers. The thermal bonding is attributed to the copolyester fiber having a lower melting point than the blended polyester fiber. An example of such a copolyester is poly(ethylene terephthalate/ isophthalate) having a terephthalate/isophthalate mole ratio from about 80/20 to 50/50.
Nonwoven products may also be formed of bicomponent fibers having a polyester core and a copolyester sheath. Such bicomponent fibers act as the binder fibers when blended with polyester fibers to make nonwovens.
The following references are directed to various DMT type processes and catalyst systems used for either making polyester or copolyester.
U.S. Pat. No. 3,709,859 to Hrach et al discloses a multi-component catalyst system for producing polyester via the ester interchange process. Among the many catalysts mentioned are lithium, cobalt, manganese and antimony. Although these catalysts are set forth in the background portion of the patent, the patent claims a catalyst system comprising antimony, lead, and calcium, and additionally strontium or barium. Hrach et al also teach the necessity of employing pentavalent phosphorous compounds as stabilizers in order to prevent the formation of discolored polyester.
British Patent No. 1,417,738 to Barkey et al discloses a process for manufacturing polyester in which a preferred ester interchange catalysts may include zinc, manganese, cobalt, and lithium, among others. Preferred polycondensation catalysts include antimony compounds. This reference, however, claims other catalyst compounds and mentions the above catalyst only as background information.
Various patents assigned to Eastman Kodak Company (British Patent Nos. 1,417,738, and 1,522,656; U.S. Pat. Nos. 3,907,754, 3,962,189, and 4,010,145) disclose a broad variety of catalyst systems, including a manganese, cobalt, lithium and titanium combination and a manganese, titanium, cobalt and antimony catalyst system, with phosphorous being used in each of these systems as a sequestering agent. Each of these catalysts was added at the beginning of ester interchange and are used as catalysts for the ester interchange. The purpose of the catalyst system is to speed up the production of the copolyester.
The following references teach preparation of a polyester by the ester interchange process with the addition of small amounts of TA in the polycondensation step utilizing various catalyst systems.
U.S. Pat. No. 3,657,180 to Cohn discloses a process for making copolyester resin in which lithium or a divalent metal compound are employed as catalyst. The specification states that manganese may be one of the divalent metallic compounds which can be employed. The order of mixing the various raw materials and the addition of the compounds to the process described in the Cohn invention is stated to be critical. The process is carried out by reacting DMT and ethylene glycol in the presence of a lithium salt under ester interchange conditions followed by the addition of manganese. In another embodiment, the process also includes using manganese as a catalyst with lithium being added after the ester interchange reaction. In either case, the second metal is always added after ester interchange, and thus is not second metal is added in a higher than catalytic amount and is added to act as a slip agent. The second metal is added along with a slurry in the amount of less than 1% of product of glycol and a small amount of terephthalic acid before vacuum-let-down to provide slip for polyester film and the amount added is several factors larger than catalytic amount.
U.S. Pat. No. 3,487,049 to Busot discloses a catalyst system of manganese, sodium and antimony. Furthermore, a small amount of terephthalic acid mixed in a glycol slurry is added to the reactor during vacuum-let-down (at 30 mm mercury) for increasing the polymerization rate, etc. Less than 4.0 percent of product weight of TA was added in this teaching.
Improvements directed to the reduction of ester interchange time or polycondensation time through the use of various catalyst systems and the addition of a very small amount of TA/glycol slurry are not particularly advantageous to the production of a copolyester through the novel sequencing of the ester interchange, direct esterification and polycondensation. In fact, although the catalyst systems disclosed in the prior art described herein may be suitable for reducing ester interchange time, they are found to differ from the catalyst system used in the present invention. As will be shown in the examples, various use of catalyst systems may produce copolyester but the quality has been found unacceptable. In particular, it has been found that the timing of the addition of the polycondensation catalysts also contribute to the production of the copolyester.
It is an aim or aspect of the present invention to not only feasibly produce a copolyester by the LDE type process from available suitable raw materials, but produce a copolyester which has acceptable color, IV and thermal properties.